tl;dr - you can install a new Seimitsu switch PCB in a PCB-less NeoGeo CD stick, for fresh feeling play. Not in the original Neo AES stick though, it has zero vertical space to spare, unless you start cutting things out. Update: the STL file attached won't allow the CD stick to be closed properly. Efforts are ongoing.
Seimitsu-01-PCB.jpg
Seimitsu's made a lot of joysticks in their long history, and they made the stick for the NeoGeo systems as well. It's effectively an LS-22 mechanism (https://nfgworld.com/rediscovering-old-joysticks/), though it's not labeled as such.
Interestingly, it's mostly compatible with Seimitsu's more modern sticks, like the SELS-70X, the LS-56, etc. Pretty much every 40mm stick has parts that are directly compatible with the NeoGeo stick.
Crucially, there are some differences. The LS-22 has a very,
very short shaft, and the base of the stick is 9mm thick where most of the newer units are 11mm.
Seimitsu-LShaft-Comparison.png
But other things, like the microswitches, can be swapped, and interestingly there are two diagonal cuts in the LS-22 base that seem intentionally made to allow the switch PCB to fit.
Seimitsu-LS-22-PerfectFit-1.jpg Seimitsu-LS-22-PerfectFit-2.jpg Seimitsu-LS-22-PerfectFit-3.jpg
And now, the point of this post:The base of the LS-22 allows a PCB-style switch assembly (https://www.cqbarcade.com/products/seimitsu-ls-56-ls-56-01-replacement-parts-collated) to be installed, in place of the original four separate microswitches. It requires a new part to hold it all in place, because the PCB has several solder points that will interfere with the original metal retaining plate. To accommodate the solder points, I designed a 3D-printable retaining plate that will hold the PCB and switches in place.
Seimitsu-LSNeoCD-Plate.jpg
There are some things to note, none of which appear to affect the functionality or durability of the mod.
Seimitsu-NeoGeo-Update-1.jpg
above: One mount hole is covered by the PCB- 2 of 4 main mounting screws won't fit anymore, because the PCB partially covers their holes
- Because of the two sets of three holes on the base, the PCB can be oriented only two ways
- The switch-retaining lumps on the metal plate are removed, but they aren't doing much work anyway
- You can't use Seimitsu's modern gates (https://www.cqbarcade.com/products/octagonal-gate?variant=41289626681404) because the LS-22 base has no receiving holes for the screws
Seimitsu-NeoGeo-Update-3.jpg
You can, of course, download the STL file and print it yourself: Seimitsu40-SNKneo-plate-1.4.stl